The measurement of the static light scattering is used for the characterisation (size, mass, form and structure) of molecules or colloidal substances. This is an absolute quantification which manages without any previous calibration or use of standard samples. A sample is illuminated with a collimated light beam, and the scattered light is measured at different scattering angles.
The principle of light scattering is widespread in nature. It can be observed, e.g. at sunset or when dust particles become visible. Light beams strike a strongly scattering medium, and are deflected from their geometrically prescribed path by particles. The intensity of the light beams is weakened here by absorption and scattering. The scattering is the basis for different physical phenomena such as e.g. deflection, refraction and reflection.
Scattering can be sub-divided into non-elastic, quasi-elastic and elastic scattering which differ in their frequency shift. With non-elastic scattering a frequency shift of approximately 1011 to 1013 Hz occurs. With quasi-elastic scattering, with which light additionally interacts with translation and rotation quanta of a molecule, a frequency shift of 10 to 106 Hz occurs. With elastic light scattering (e.g. static light scattering) there is no change to the wavelength (also called coherent scattered radiation). The underlying principle of light scattering can be demonstrated with a very small, optically isotropic gas molecule. The electrons of the molecule are caused to vibrate through the incidence of the electromagnetic wave with the frequency of the stimulating light source. The oscillating dipole thus produced in turn emits electromagnetic radiation of the same frequency, the intensity of the radiation depending on the strength of the induced dipole, i.e. the more polarisable the molecule, the stronger the dipole and the greater the intensity of the emitted radiation.
If a sample, for example a suspension, in which a number of macromolecules are located, is illuminated with a collimated light beam, every macromolecule emits radiation. The sum of the intensities of the emitted radiation is in proportion to the concentration of the macromolecules in the suspension and of the molar mass of the molecules. Furthermore, the size of the molecules contained in the colloid can be calculated from the angle dependency of the scattered light intensities since the light scattered at the different scattering centres in the macromolecule interferes and produces an angle-dependent scattering pattern. The average values of the particles located in the cell are respectively determined here. In the prior art measuring instruments are described which measure the scattering properties of colloidal liquids and use them for the characterisation of the material properties. EP 0 182 618 B1 discloses a device which describes the measurement of the static light scattering by means of a sample cell. The sample cell can be coupled to a chromatographic construction so that the particles, separated according to size, flow through the sample cell. For this a round glass cell is provided with a longitudinal bore hole through which a flow of liquid with the contained particles is fed and is illuminated with a laser beam. Detectors, which collect the scattered light, are arranged around the round glass cell at different angles. In order to determine the angle dependency every detector may only collect a small angle range. Therefore, with this instrument it is necessary to reduce the detected range in the bore hole to a few nanoliters by means of apertures, and this increases noise and interference. This inevitably leads to a reduction in sensitivity.
This technology was described first of all in U.S. Pat. No. 4,616,927 and in EP 0 182 618. However, they only disclose the measurement of the scattered light at a number of different angles. The scattering range observed is limited to a few nanoliters by means of apertures. EP 0 626 064 is a further development where measurements are taken at 2 angles, the light scattered at 15 degrees being collected by means of a lens and aperture system.
In U.S. Pat. No. 6,052,184 the scattered light is collected by means of fibre optics, the latter also only observing a very small liquid range however. The flow of liquid is guided here perpendicularly to the incident light beam. In EP 1 515 131 it is described how the volume of liquid can be minimized by using a second flow of liquid, the volume of liquid observed also being limited, however.
It is a disadvantage with the devices disclosed in the prior art that the volume of liquid observed is extremely limited by aperture systems or the use of fibre optics which are brought close to the scattering centre in order to obtain a good angle resolution and to keep scattered light away from air/glass/medium interfaces. Therefore the sensitivity of the method is reduced.